fix(sync): R5 S2 — sync guards

- never prune subscriptions on an empty fetch: one anomalous empty-but-200
  response wiped every subscription (and its deep_requested flag) in one run
- push_playlist: stop after 5 consecutive write failures — a quota-dead or
  scope-revoked account otherwise burns 50 units per remaining item
- RSS: raise RssError instead of returning [] on a transport/HTTP failure, so a
  failed poll no longer stamps last_rss_at and a 404 feed stops looking healthy
- worker: exit when the schema never appears instead of "continuing" into a
  crash one call later

backend/tests/test_sync_guards.py

@@ -0,0 +1,28 @@
+"""Guards that stop one bad YouTube answer from cascading: the playlist-push circuit breaker."""
+from app.sync.playlists import _Breaker
+
+
+class TestBreaker:
+    def test_open_until_the_limit_is_reached(self):
+        b = _Breaker(limit=3)
+        for _ in range(2):
+            b.hit(False)
+            assert b.open()
+        b.hit(False)
+        assert not b.open() and b.tripped
+
+    def test_a_success_re_arms_the_streak(self):
+        b = _Breaker(limit=3)
+        b.hit(False)
+        b.hit(False)
+        b.hit(True)  # one write went through — this isn't an account-wide failure
+        b.hit(False)
+        b.hit(False)
+        assert b.open()
+
+    def test_stays_tripped_once_tripped(self):
+        b = _Breaker(limit=2)
+        b.hit(False)
+        b.hit(False)
+        b.hit(True)
+        assert not b.open()